Projective displays use images focused onto or through a diffuser, screen, lens, or other optic module to present an image to a user. The projection may be done from the same side of the optic module as the user (i.e., reflecting the image off the optic module), as in the case of cinema projectors and a cinema screen, or from the opposite side, such as is used in current rear projection television technology. Prior art projection systems typically produce a two-dimensional image by using a single projector to project a single image at a time onto a screen. The screen would have a linear light distribution such that all viewers of the image reflected by screen would see the same two-dimensional image regardless of the positions at which they are located.
In contemporary projection systems, the image can be generated on one or more “displays,” such as a miniature liquid crystal display device that reflects or transmits light in a controllable pattern formed by its constituent switchable pixels. Such liquid crystal displays are generally fabricated with microelectronics processing techniques such that each grid region, or “pixel,” in the display is a region whose reflective or transmissive properties can be controlled by an electrical signal. In a liquid crystal display, the light incident on a particular pixel is either reflected, partially reflected, or transmitted by the pixel, depending on the signal applied to that pixel. In some cases, liquid crystal displays are transmissive devices where the transmission through any pixel can be varied in steps (gray levels) over a range extending from a state where light is substantially blocked to the state in which incident light is substantially transmitted. Such transmissive liquid crystal displays can be used as controllable optic modules as is known in the art.
When a uniform beam of light is reflected from (or transmitted through) a liquid crystal display, the beam gains a spatial intensity profile that depends on the particular transmission state of the various pixels in the display. Thus, an image can be formed at the liquid crystal display by electronically adjusting the transmission (or gray level) of each of the pixels to correspond to a desired image. This image can be projected (such as by backlighting a transmissive liquid crystal display) onto a diffusing screen for direct viewing, or, alternatively, it can be imaged through a lens or some other optic module by which it can be magnified by an eyepiece or otherwise viewed to give a virtual image.
The three-dimensional display of images, which has long been the goal of electronic imaging systems, has many potential applications in modern society. For example, training of professionals, from pilots to physicians, now frequently relies upon the visualization of three-dimensional images. Understandably, three-dimensional imaging also has numerous potential applications in entertainment. In many applications of three-dimensional imaging it is important that multiple perspectives of an image be able to be viewed so that, for example, during simulations of examination of human or mechanical parts, a viewer can have re-created a continuous three-dimensional view of those parts from multiple angles and viewpoints in a manner closely emulating real life three-dimensional vision.
Thus, real-time, three-dimensional image displays have long been of interest in a variety of technical applications. Heretofore, several techniques have been known in the prior art to be used to produce three-dimensional and/or volumetric images. These techniques vary in terms of complexity and quality of results, and include computer graphics which simulate three-dimensional images on a two-dimensional display by appealing only to psychological depth cues; stereoscopic displays which are designed to make the viewer mentally fuse two retinal images (one each for the left and right eyes) into one image giving the perception of depth; holographic images which reconstruct the actual wavefront structure reflected from an object; and volumetric displays which create three-dimensional images having real physical height, depth, and width by activating actual light sources of various depths within the volume of the display.
Auto-stereoscopic displays in particular have been widely researched in attempts to create real-time full-color three-dimensional displays. The principle of stereoscopy is based upon the simultaneous imaging of two different viewpoints, corresponding to the left and right eyes of a viewer, to produce a perception of depth from paired two-dimensional images. In stereoscopic imaging, an image is recorded using conventional photography (or video recording) of the object from different vantages that correspond, for example, to the distance between the eyes of the viewer.
Ordinarily, for the viewer to receive a spatial impression from viewing stereoscopic images of an object projected onto a screen, it has to be ensured that the left eye sees only the left image and the right eye only the right image. While this can be achieved with headgear or eyeglasses, auto-stereoscopic techniques have been developed in an attempt to abolish this limitation. Conventionally, however, auto-stereoscopy systems have typically required that the viewer's eyes be located at a particular position and distance from a view screen (commonly known as a “viewing zone”) to produce the stereoscopic effect.
One way of increasing the effective viewing zone for an auto-stereoscopic display is to create multiple simultaneous viewing zones. This approach, however, imposes increasingly large bandwidth requirements on image processing equipment. Furthermore, much research has been focused on eliminating the restriction of viewing zones by tracking the eye/viewer positions in relation to the screen and electronically adjusting the emission characteristic of the imaging apparatus to maintain a stereo image. Thus, using fast, modern computers and motion sensors that continuously register the viewer's body and head movements as well as a corresponding image adaptation in the computer, a spatial impression of the environment and the objects (virtual reality) can be generated using stereoscopic projection. As the images become more complex, this prior art embodying this approach has proven less and less successful.
The nature of stereoscopic vision allows parallax to be observed only from discrete positions in limited viewing zones in prior art auto-stereoscopy systems. For example, any stereoscopic pair in standard auto-stereoscopy systems gives the correct perspective when viewed from one position only. Thus, auto-stereoscopic display systems must be able to sense the position of the observer and regenerate the stereo-paired images with different perspectives as the observer moves. This is a difficult task that has not been mastered in the prior art.
Further, there is also present interest in various industries for relatively inexpensive systems that can generate relatively high quality “free space” images in free space whereby the image appears to float in air in front of the viewer without requiring the user to wear specialized eyewear or any such implements. U.S. Pat. No. 6,055,100, issued to Kirk et al., discloses one particular free space imaging system. This particular system utilizes a doublet form of optic module, where the doublet is formed from two large Fresnel lenses configured such that their echelon groove components are mutually oppositely disposed within the optical transmission path of the system. The projection of an image through the lens doublet to a focal point on the other side of the doublet causes a predictable curving to the output focal plane for the projected image.
While the use of lens doublet as disclosed by Kirk emulates a sense of three-dimensionality to a viewer within a relatively wide field of view, this system still is not ideal for three-dimensional free-space imaging. The system disclosed by Kirk attempts to create a three-dimensional viewing effect using a projected two-dimensional image and chromic aberrations. This approach leads to an imperfect image effect.
In light of the current state of the art of image projection, it would be desirable to have a system that is capable of projecting numerous aspects or “multi-aspect” images such that the user can see many aspects and views of a particular object when desired. It would further be useful for such viewing to take place in a flexible way so that the viewer is not constrained in terms of the location of the viewer's head when seeing the stereo image. Additionally, it would be desirable for such a system to be able to provide superior three-dimensional image quality while being operable without the need for special headgear. Thus, there remains a need in the art for improved methods and apparatuses that enable the projection of high-quality three-dimensional “free space” images to viewing locations.